Enhancing the performance of distance protection relays under practical operating conditions.

152 Figure 7.5: A-g, B-g and A-B impedance plots for relay 1a for a cross-country fault at 95% of the line; ideally transposed lines with no interconnection, both transmission lines in service and no POTT scheme used. 153 Figure 7.8: A-g, B-g and A-B impedance plots for relay 2b for a cross-country fault at 95% of the line; ideally transposed lines with no interconnection, both transmission lines in service and no POTT scheme used.

Background and importance of research

Another phenomenon that will be investigated is the effect of the untransposed nature of many of the transmission lines in parts of South Africa. This issue can again be addressed by a thorough real-time simulator study using practical protection relays and detailed modeling of the relevant transmission plant.

Research question

Aims and objectives

Methodological approach

Thesis layout

The theoretical rationale for the effects of non-transposed transmission lines will be presented with derivations from first principles. Then, the scheme is tested under a very challenging operating condition that combines the effects of mutual coupling between transmission lines and untransposed transmission lines with the presence of a cross country fault on the system.

Research publications

Requirements of a protection scheme

Reliability: This is the most important protection requirement and implies the ability of the protection system to always function when needed. What matters is the ability of the protection scheme to remain stable to out-of-zone faults.

Faults

Selectivity (or Discrimination): This involves accurately identifying the location of the fault and selectively isolating the fault, or the part of the system experiencing the fault, from the rest of the network. Sensitivity: This is the ability of the protection scheme to detect incipient faults (low level faults) as well as large high current faults without compromising the four requirements of the protection scheme mentioned above.

Transmission line electromagnetic field

These categories of faults lead to three phase faults, single phase to ground faults, phase to phase faults, phase to phase to ground faults and cross country faults [11].

Basic transmission line mathematical models

D is the distance between the center of the two conductors a is the radius of the conductor. D is the separation between the centers of the two conductors a is the radius of the conductor.

Distance protection relays

This k0 is needed if the relay manufacturer uses the positive sequence impedance of the line for the calculation of apparent impedance [16]. 𝑍𝐿1 is the positive sequence impedance of the line 𝑍𝐿2 is the negative sequence impedance of the line 𝑍𝐿0 is the zero sequence impedance of the line.

Figure 2.4: Sequence network for an A-g fault In Figure 2.4: 𝐼 1 is the positive sequence current 𝐼 2 is the negative sequence current 𝐼 0 is the zero sequence current

Mho impedance diagrams

As can be seen in Figure 2.10, this impedance corresponds to the impedance of the line between the relay location and the fault point (𝑍𝐹1). Remote protection depends on the relay's operating characteristics as defined by the user.

Figure 2.9: Impedance plot

Resistive ground faults

Due to the effect of the resistance of the fault, the relay will see the apparent impedance OB, and therefore the fault at point B, in zone 2. This has the effect that the fault resistance seen by the relay is much larger as the actual fault resistance, i.e.

Permissive signalling

For F3, the relay at A will see a fault in both its zone 1 and 2, and the relay at B will see a fault in its zone 2. Relay A will now see a fault in zone 2 and in addition receive an allowable signal sent to relay B.

Figure 2.13: Permissive over-reaching transfer trip scheme

Double circuit and parallel transmission lines

Mutual coupling

Where: 𝐼𝑓𝑎𝑢𝑙𝑡 is the fault current on the protected line k0 is the zero sequence current compensation factor 𝐼0 is the zero sequence current of the protected line 𝑘0𝑀 is the mutual coupling factor. 𝑍1𝐿 is the positive sequence impedance of the line. 𝑍0𝐿 is the zero sequence impedance of the line.

Figure 2.15: Mutual coupling between two conductors

Transposition of transmission lines

The effects of transmission line failure are well understood, but the effect on distance protection relays is not widely documented. A complete mathematical investigation of unbalance on a distance protective relay will be discussed in the fifth chapter of this thesis.

Cross country faults

An a-g fault on the first transmission line will cause the fault from current on the second transmission line to be carried through the busbar. At the same time, if a B-g fault occurs on the second transmission line, the fault current from the current on the first transmission line will be carried through the connecting rail.

Figure 2.26: Current distribution of a cross country fault in parallel feeders

SEL 421 relay

The SEL 421 Remote Protection Relay provides the necessary capabilities for a remote protection scheme and will be used in the various studies performed in this assignment where hardware protection relays are involved. Distance protection mho works for phase-to-phase, phase-to-earth and three-phase faults.

Figure 2.28: Relay to relay communication [48]

The real time digital simulator

Any of the three SELogic control equations if enabled. A very important application of RTDS is the closed-loop testing of protective relays and integration into a power system [51].

Figure 2.32: Pictorial overview of the integration of hardware and software

RSCAD simulation model of a distance protection relay

In this way, the RSCAD software relay was used to provide an indication of the impedance seen by the SEL 421 relay during detailed test studies.

Study system model for the research studies

For the study, the section of transmission line between Mersey and Impala was modeled according to the program RSCAD DRAFT. This error control logic predicts that more than one type of error is placed in the system at the same time at any two points in the network.

Figure 2.37: Transmission line topology to be studied

Conclusion

The next chapter will continue to derive the equations for calculating the impedances of a double circuit and a parallel transmission line where there is mutual coupling between the transmission lines. Protection of transmission lines today is performed by protective relays that make use of the positive and zero sequence impedances of the transmission lines in their decision algorithms.

Derivation from first principles

The above theory is essential to the procedure used in calculating the impedance of mutually coupled transmission lines. This mathematical representation of the transmission line includes the magnetic coupling between each pair of conductors and in the format in eqn.

Figure 3.2: Three phase transmission line with ground replaced by ground return conductors

Calculation of mutual impedances between parallel transmission lines and verification

The self-impedances of the ground lines are then calculated using the parameters in Figure 3.8 and Eq. The self-impedances of the ground wires are then calculated using the parameters in Figure 3.12.

Figure 3.5: Phase conductor data

Conclusion

When no interconnection between the parallel lines is shown, each line is modeled using its own resulting 3x3 phase impedance matrix equation according to eqn. When the mutual coupling between the lines is shown, a single 6x6 matrix ZP is obtained representing the two three-phase systems as a unitary, coupled electric and magnetic system, 𝐸𝑃𝐿1𝐿2 = 𝑍𝑃𝐿1𝐿2𝐼𝑃𝐿1𝐿2.

Setup of model used to research mutual coupling between transmission lines

In the investigations carried out in this chapter of the thesis, the transmission lines were assumed to be ideally transposed. From the DRAFT model in Figure 4.2, note that the parallel transmission lines in the upper part of the figure are independent of each other (two distinct transmission line models with distributed parameters), and the parallel transmission lines in the lower part of the figure are represented using coupled circuit transmission line tower symbols (a combined distributed-parameter transmission line model used to represent the parallel lines).

Figure 4.2: DRAFT model of the power system without mutual coupling (above) and with mutual coupling (below)

Results

The error in range 1 of the relay range at A was then plotted as a function of the distance between the transmission lines, and the results are shown in Figure 4.12. The error in range 1 of the relay range at A was then plotted as a function of the distance between the transmission lines, and the results are shown in Figure 4.18.

Figure 4.9: Impedance measured by the C phase ground element of the relay at A for a C-g fault located at 80% along the line AB with no mutual coupling between the lines

Mutual coupling compensation

The impedance plot on the right in Figure 4.19 is the response of the RSCAD relay model to the same fault under the same operating condition, but when mutual coupling compensation is used in the relays. Therefore, this method of compensating the relay for the effects of mutual coupling compensation is effective.

Figure 4.19 shows the effectiveness of this method when both lines are in service. A phase C to ground fault was placed at 80% along transmission line A-B when both parallel transmission lines were in service, and the impedance plot for this case is show

Conclusion

The most effective and safe approach is to adjust the range of the protection zones according to the extent of mutual coupling for the particular wire being protected. In this way, not only the impact of mutual coupling on a particular power system and a particular relay's algorithms can be accurately studied, but also its impact on a full protection relay scheme can be evaluated directly on the physical protection relays.

Testing phase element reach errors using hardware and software relays

The effect of untransposed transmission lines on permissive distance protection trip

If we first examine the effect on the elements of zone 2, where the range is set to 120%, and use the phase C-A fault at 120% of the line, the impedance plot for the phase C-A impedance loops of the RSCAD software relays is shown in Figure 5.11. From the impedance diagrams in Figure 5.11, it can be seen that for a non-transposed line with a fault at 120% of the line, the protection of the transmission line may no longer be achieved.

Figure 5.12: Impedance plots for a phase C-A fault at 112.5% of a line with ground wires: left plot – Relay 1 protecting the ideally transposed line; right plot – Relay 3 protecting the untransposed line

Conclusion

The individual effects of mutual coupling between parallel transmission lines and non-transposition of transmission lines on the distance protection relay have been presented from first principles in Chapters 3, 4 and 5. This chapter will examine a more complex operating condition where the effects of mutual coupling and untransposed transmission lines are considered together in the same survey system.

Background

In order to investigate the combined effects of transmission line interconnection and non-transposition, the types of faults that would be placed on the network had to be carefully thought out so that both phenomena could be fully represented and studied together. In the second parallel transmission line system, the two transmission lines were modeled using a single distributed parameter model, so that the mutual coupling was fully represented between the conductors of the two transmission lines and both transmission lines were shown as non-transposed.

Three phase to ground fault at 100% of the transmission line

The response of an impedance protection relay is shown in Figures 6.3 and 6.4 if we use exactly the same three-phase earth faults in the system, but now with mutual coupling between the transmission lines and with the non-transmitted transmission lines. Note now that due to the combined effects of mutual coupling (which only affected the phase-to-earth impedance loci for earth faults) and transmission line non-transmission (which mainly affected the phase-to-earth impedance loci for phase faults) both the phase-to-phase impedance loci and earth seen by the distance protection relay show noticeable changes.

Figure 6.1: Phase to ground impedance plots for a three phase to ground fault at 100% of the line with ideally-transposed transmission lines and no mutual coupling present between the transmission lines with both transmission lines in service

Impedance plots for this scenario are shown in Figures 6.9 and 6.10 with an AB-g fault set to 100% of the line. Impedance plots for this condition are shown in Figures 6.11 and 6.12 for AB-g fault at 100% line.

Figure 6.7: Phase to ground impedance plots for an AB-g fault at 100% of the line with ideally transposed transmission lines and no mutual coupling present between the lines with both lines in service

Three phase to ground fault at 0% of the transmission line

It is important to note that the closer the faults are to the remote end, the worse the effect of the cross-country fault on the remote protection relay on the sending side. The interface between the SEL 421 hardware relay and the RSCAD model for the tests of the POTT2 scheme is shown in Figure 7.36.

Figure 6.19: Phase to ground impedance plots at relay B for a three phase to ground fault at 0%

Fault involving ground resistance and resistance between the phases of the transmission line

Conclusion

The studies presented in this chapter considered the combined effects on distance relays of the complex, practical operating conditions that may be present in typical parallel transmission lines. The next chapter will explore further challenging conditions that parallel transmission lines may encounter in practice in the form of cross faults.

Investigating the effects of a cross country fault on the distance protection relay

In Figure 7.10, the blue arrows show the directions in which the actual current measurement on each relay is directed from forward areas 1 and 2. For this fault, the relays at the receiving end (relays 1b and 2b) issue a single pole trip and the relays at the transmitting end trip three-pole tripping 400 ms after the fault is placed in the network.

Figure 7.5: A-g, B-g and A-B impedance plots for relay 1a for a cross country fault at 95% of the line; ideally transposed lines with no mutual coupling, both transmission lines in service and no POTT scheme used

Investigating the effects of a cross country fault on the distance protection relay with a

At the moment the fault is located on the system, relay 1b emits an instantaneous single-pole trip and relay 1a picks up on A-phase. When the B-g fault appears on line 2, relay 2b delivers an instantaneous single-pole trip and relays 1a and 2a pick up on B phase.

Figure 7.21: A-g, B-g and A-B impedance plots for relay 1a for a cross country fault at 95% of the line; ideally transposed lines, no mutual coupling present, both transmission lines in service and a simple POTT scheme used

Investigating the effects of a cross country fault on the distance protection relay with a

For example, if the transmitting limit relay receives KEY 1 permission to trip and identifies the fault as a single-phase to ground fault, the relay will initiate a single-pole trip. BRKx – signal status for circuit breaker at point x (1a, 1b, 2a or 2b) showing the binary to decimal status of the circuit breaker.

Figure 7.26: Additional POTT2 scheme logic designed using RSCAD library components

Investigating the effects of a cross country fault on the distance protection relay with a

The relay at the transmitting end clearly sees the fault as a fault involving more than one phase, as the tripping permission to the remote end is sent to channel 1 and channel 2 of the POTT2 protection scheme. Note that although the sender end relay detects a polyphase fault because it receives a trip permission from the far end only on channel 1, it does not immediately issue a three-pole trip command because it did not receive a trip permission on channel 2, indicating that a fault involving only one phase has occurred before the remote end relay.

Investigating the effects of a cross country fault with the addition of mutual coupling and

The next section examines the response of the POTT 2 scheme and distance protection relays in the presence of cross faults when the parallel transmission lines are untransposed and mutually connected. Investigating the effects of a cross-site fault with the addition of interconnect and non-transposed transmission line.

Investigating the effects of a cross country fault involving phase a and c with mutual

Investigating the effects of a cross country fault closer to the zone 1 reach where mutual

Conclusion